A resistively operating nonvolatile memory cell has at least two different electrical resistances which can be assigned, for example, to the states “0” and “1”. The memory cell may have a higher or lower electrical resistance depending on the applied voltage and can be switched between these two resistances.
One of the main aims in the further development of modern memory technologies is to increase the integration density, which means that it is very important to reduce the feature size of the memory cells on which the memory devices are based.
The technologies used, such as for example DRAM, SRAM or flash memories, have various drawbacks, such as for example volatility (DRAM), size (SRAM) or low endurance (number of possible write/read cycles). Hitherto, there has been no single technology that has been able to satisfy all the requirements for various applications.
Ionic solid-state memories are one of the highly promising technologies for nonvolatile memory cells. By way of example, it is known that certain metals, such as for example silver or copper, can be dissolved in chalcogenide glasses. The term glass is to be understood in very general terms as an amorphous, cooled melt, the atoms of which do not have a continuous long-range order, but rather only a locally limited crystalline arrangement (short-range order) in a three-dimensional unordered network.
Glasses may fundamentally be both electrical insulators, metals and electrical semiconductors, depending on whether free charge carriers are present and what ions are present in the glass and also on whether the ions that are present are able to move or are bonded. The conductivity of the chalcogenide glasses can be obtained, for example, by ions, such as for example sodium, lithium or silver ions, being incorporated in the glass. If metal ions are dissolved in the glass, the system can be considered as a “solid-state electrolyte” and the glass alone can be considered as a “solid-state ion conductor”.
Chalcogenide glasses can be produced on the basis of the compounds of the general formula MmXn, where M denotes one or more elements or metals selected from the group consisting of group IVb of the periodic system, group Vb of the periodic system and transition metals, X denotes one or more elements selected from the group consisting of S, Se and Te, and m and n have a value of between 0 and 1. The indices m and n do not have to be integer numbers, since metals and elements may have a number of oxidation states present simultaneously. In accordance with the present invention, groups IVb and Vb are to be understood as meaning main groups IV and V, respectively, of the periodic system (groups 14 and 15 according to the new IUPAC nomenclature). The preferred elements or metals of these groups are Ge, Sn, Pb, As, Sb, Bi and Si.
One promising approach for the fabrication of resistive nonvolatile memory cells is based on the use of the solid solutions in chalcogenide glasses as active (switching) material for nonvolatile memory cells. A memory cell of this type has a layer of a chalcogenide glass arranged between a first electrode and a second electrode. In the simplest case, metal ions of the material forming one of the electrodes are dissolved in the layer of a chalcogenide glass.
Chalcogenide glass memory cells are based on an electrochemical redox process, in which metal ions of one electrode can reversibly diffuse into and out of the solid-state electrolyte material, thereby forming or dissolving a low-resistance path. More specifically, the material comprising chalcogenide glasses is arranged between two electrodes, with one electrode being designed as an inert electrode and the other electrode being designed as what is known as a reactive electrode. The ions of the reactive electrode are soluble in the chalcogenide glass.
The chalcogenide glasses are generally semiconductive. The dissolving of the metal ions in the chalcogenide glasses produces a solid solution of the corresponding ions in the glass. Silver ions can, for example, be dissolved by the deposition of an Ag film on a chalcogenide glass and subsequent irradiation. The irradiation of a sufficiently thick Ag film on Ge3Se7 produces, for example, a material of formula Ag0.33Ge0.20Se0.47. Accordingly, the solutions may form by the photo-dissolution of silver in, for example, As2S3, AsS2, GeSe2.
In Hirose et al., Journal of Applied Physics, Vol. 47, No. 6, 1976, pp. 2767 to 2772, “Polarity-dependent memory switching and behavior of Ag dendrite in Ag-photodoped amorphous As2S3 films,” an arrangement is described including an inert electrode of molybdenum or gold, a second electrode of silver and a layer of a chalcogenide glass of As2S3 photo-doped with Ag+ ions arranged between the two electrodes. Applying a positive voltage to the Ag electrode, which voltage must be higher than what is known as a minimum threshold voltage, oxidizes the electrode, forces the Ag+ ions into the chalcogenide glass and reduces them again in the solid-state electrolyte material or on the inert electrode, which leads to metallic deposits in the form of a conductive metal-rich path between the first and second electrodes. This lowers the electrical resistance of the arrangement. In this state, the electrical resistance of the solid-state electrolyte is reduced significantly (for example by several orders of magnitude) compared to the state without a metallic current path, thereby defining the ON state of the memory cell. If an oppositely polarized voltage is applied to the two electrodes, this leads to the formation of the metallic deposits or the current path being reversed, with the result that the two electrodes are no longer continuously electrically connected to one another, thereby defining the OFF state of the memory cell, since in this state the memory cell has a higher resistance than in the ON state.
Therefore, the general mechanism can be explained as being that the reactive electrode together with the solid-state electrolyte forms a redox system in which a redox reaction takes place above a defined threshold voltage (Vth). Depending on the polarity of the voltage which is applied to the two electrodes, although this voltage must be higher than the threshold voltage, the redox reaction can take place in one reaction direction or the other. Depending on the applied voltage, the reactive electrode is oxidized and the metal ions of the reactive electrode diffuse into the chalcogenide glass and are reduced in the solid-state electrolyte material or at the inert electrode. If metal ions are being continuously released into the solid-state electrolyte, the number and size of the metallic deposits increase until ultimately a metallic or semiconducting low-resistance current path which bridges the two electrodes is formed (ON state). The electrolytic deposition ceases when so much metal has diffused in that the threshold voltage Vth is undershot. If the polarity of the voltage is reversed, metal ions diffuse out of the chalcogenide glass and are reduced at the reactive electrode, which causes the metallic deposits located on the inert electrode to break down. This process is continued under the influence of the applied voltage until the metallic deposits which form the electrical path have been completely broken down (OFF state). The electrical resistance of the OFF state may be 2 to 6 orders of magnitude greater than the resistance of the ON state.
The implementation of individual switching elements which are based on chalcogenide glasses, such as As2S3, GeSe or GeS and WOx, is known and published, e.g., in M. N. Kozicki et al., “Superlattices and Microstructures”, Vol. 27, No. 5/6, 2000, pp. 485-488, M. N. Kozicki et al., Electrochemical Society Proceedings, Vol. 99-13, 1999, pp. 298-309, “Applications of Programmable Resistance Changes in Metal-Doped Chalcogenides” and M. N. Kozicki et al., 2002, “Can Solid State Electrochemistry Eliminate the Memory Scaling Quandary?”. 
The above-referenced publications propose depositing the solid-state electrolyte in a via hole (hole between two metallization levels of a semiconductor element) which has been etched vertically in a conventional inter-dielectric. Then, the material of the reactive electrode is deposited and patterned, for example by a suitable etching process or by chemical mechanical polishing (CMP). This is followed by a process which drives the material of the reactive electrode into the solid-state electrolyte in order to generate background doping of the solid-state electrolyte with the metal of the reactive electrode by UV irradiation.
However, the implementation of the memory cells based on the abovementioned chalcogenide materials brings with it serious problems, for example the fact that the limited thermal stability of the chalcogenide glasses requires special measures for back-end integration of a fully integrated memory. By way of example, Se-rich GeSe has a phase change at just 212° C., which throws up serious problems in particular for processing in the back-end sector (e.g., see Gokhale et al., Bull, Alloy Phase Diagrams 11 (3), 1990).
Hitherto, chalcogenide layers of the binary composition MmX1-m, where M denotes one or more elements or metals selected from the group consisting of group IVb of the periodic system, group Vb of the periodic system and transition metals, X denotes one or more elements selected from the group consisting of S, Se and Te, and m has a value of between 0 and 1, have been produced by evaporation coating processes (see, e.g., Petkova et al.: Thin Solid Films 205 (1991), 205; Kozicki et al.: Superlattices and Microstructures, Vol. 27, No. 5/6 (2000) 485-488) or by a sputtering process using suitable sputtering targets, such as for example by multi-source deposition (see, e.g., E. Broese et al., Journal of Non-Crystalline Solids (1991), Vol. 130, No. 1, p. 52-57), alloying targets (see, e.g., Moore et al., Physics of Non-Crystalline Solids, Taylor & Francis, London, UK, 1992, p. 193-197, xvi+761 pp 7 ref.; Conference: Moore et al.: Conference paper (English), Cambridge, UK, 4-9 Aug. 1991 ISBN 0-7484-0050-8, M. W.; France et al. Sputtering of Chalcogenide coatings on the fluoride glass) or by a multi-component target (see, e.g., Choi et al.: Journal of Non-Crystalline Solids Elsevier: 1996, Vol. 198-200, pt. 2, p. 680-683, Conference: Kobe, Japan 4-8 Sep. 1995 SICI. 0022-3093 (1995605) 198/200: 2L. 680: OPSU; 1-8 ISSN 0022-3093 Conference paper (English), P Optical properties and structure of unhydrogenated, hydrogenated, and zinc-alloyed GexSe1-x films prepared by radiofrequency sputtering).
Since the compounds of the composition MmX1-m are completely miscible in the amorphous phase over the concentration range, it is possible to determine the composition by suitable selection of the material or sputtering target which is to be vaporized. The most important of these processes is sputtering deposition of these binary chalcogenide layers (e.g. Ge—Se or Ge—S) from a binary mixed target. The doping of the chalcogenide layer with nitrogen has only been described for ternary chalcogenide layers. For example, in Kojima et al.: “Nitrogen Doping Effects on Phase Change Optical Discs, Jpn. J. Appl. Phys. Vol. 27 (1998), p. 2098-2013), the effects of the chalcogenide layers caused by doping with nitrogen were investigated with regard to the optical properties of a recording medium.
One significant difficulty of the abovementioned chalcogenide materials is, as has already been mentioned above, an inadequate thermal stability for direct integration in a standard CMOS back-end-of-line (BEOL) process. The abovementioned binary layers, for a CMOS process, have to be treated at a temperature of approximately 430° C. for several hours, and therefore have to remain stable in the amorphous state during this time. However, it has been determined that a Ge—Se layer starts to form crystalline nuclei at temperatures of even just 150° C., and at 350° C. a matrix of Ge—Se compounds (with Ag+ ions dissolved therein) crystallizes completely.